Swimming with the mycobacteria

The patients were working out in a hospital's warm therapeutic pool in Boulder, Colorado, when Mark Hernandez walked in and dipped sterile bottles in the water.

By Cathryn Delude | June 6, 2005

The patients were working out in a hospital's warm therapeutic pool in Boulder, Colorado, when Mark Hernandez walked in and dipped sterile bottles in the water. He also set up a liquid impinger, a device that suctions air from above the pool and entrains the particulates, including microorganisms, in sterile buffered water. The swimmers, seemingly unperturbed, continued their physical therapy.

The hospital had called Hernandez, an environmental engineer at the University of Colorado, Boulder, who specializes in water disinfection systems, to solve a worrisome problem. Several pool workers had become sick with persistent respiratory afflictions often caused by Mycobacterium avium. But their monitoring and lab tests hadn't detected these bacteria in the water.

What Hernandez found back at the lab, however, might have given pause to the swimmers watching him set up. Under the microscope, the pool water was found to be loaded with 10 to 100 times more microorganisms than the drinking water used to fill the pool.

"When I saw those numbers, I knew something was wrong," Hernandez recalls. "But it makes sense. You've got a tremendous bather load; you're putting hair, sweat, and all kinds of organic carbon in there. The warm water enriches microbes that thrive at body temperature, and conventional disinfectants behave differently when temperatures increase."

In findings reported earlier this year in PNAS (102:4860–5, March 29, 2005), he and colleague Norman Pace found that potentially pathogenic mycobacteria accounted for more than 30% of the microbes in the water and 80% of those in the air above the pool. Mycobacteria's waxy cell wall of mycolic acid makes them hydrophobic, so they cling to bubbles created by the pool's jets that released into the air because of the heat of the water. "More of the nonhydrophobic bacteria stay in the water," he says. The same process happens in hot tubs, giving rise to "hot-tub lung" infections, which the Center for Disease Control and Prevention has tracked.

"It's the first documentation of preferential partitioning of mycobacteria into aerosols," says Pace, who exudes a boyish enthusiasm that his postdocs find "infectious." Pace, recipient of a 2001 MacArthur Foundation "genius grant," which he promptly divided amongst his graduate students, is a workaholic who has revealed the microbial life of places as mundane as shower curtains and as exotic as Yellowstone hot springs.

Hernandez and Pace found the elusive bacteria because, instead of using the hospital's traditional culture methods, they used a molecular technique that Pace developed, which relies on amplification of ribosomal RNA genes. "It's quite remarkable that essentially our entire understanding of microorganisms has been based on cultures," he says, "when it turns out that we can't culture much of what's out there," including M. avium, which is notoriously slow-growing and difficult to culture.

Pace identifies the organisms by comparing "barcode" sequences of those genes with those of known organisms. "Use of these methods has expanded dramatically the known extent of microbial diversity," Pace says. "Two thirds of the known major bacterial phyla have no cultured representation and were discovered using the sequencing approach." Pace believes standard culturing misses more than 99% of the organisms in the environment.

The hospital has installed new air filters above the pool and zaps the water once a month with high doses of chlorine, which have alleviated the problem, Hernandez says. For your peace of mind, it's best not to ask Pace what he's found on shower curtains.